This study investigated adolescents' developmental trajectories of mathematics interest and explored related effects of gender, family, and school context. Latent growth curve modeling was used to analyze longitudinal data of N=3,193 students (51% female) from grades 5 to 9 from all 3 ability tracks of the German state school system. Annual assessments involved student questionnaires on interest in mathematics, perceptions of classroom characteristics (classroom values for mathematics, mathematics teacher enthusiasm), as well as parent questionnaires regarding family values for mathematics. Results indicated a downward trend of students' mathematics interest that plateaued in later years, with high variability in mean levels, but little variability in the shape of the growth trajectories. Boys reported higher mathematics interest than girls, but similar downward growth trajectories. Students from the lowest ability track showed more favorable interest trajectories than students from the middle and highest tracks. Family values and classroom characteristics were positively related to within-person levels of interest over time and to average individual levels of interest, but not to growth parameters. Theoretical and practical implications are discussed.